1. Field of the Invention
The present invention relates to a soluble GlcNAc phosphotransferase, a method of making the same and a method of phosphorylating with the same.
2. Discussion of the Background
Lysosomes are organelles in eukaryotic cells that function in the degradation of macromolecules into component parts that can be reused in biosynthetic pathways or discharged by the cell as waste. Normally, these macromolecules are broken down by enzymes known as lysosomal enzymes or lysosomal hydrolases. However, when a lysosomal enzyme is not present in the lysosome or does not function properly, the enzymes specific macromolecular substrate accumulates in the lysosome as “storage material” causing a variety of diseases, collectively known as lysosomal storage diseases.
Lysosomal storage diseases can cause chronic illness and death in hundreds of individuals each year. There are approximately 50 known lysosomal storage diseases, e.g., Pompe Disease, Hurler Syndrome, Fabry Disease, Maroteaux-Lamy Syndrome (mucopolysaccharidosis VI), Morquio Syndrome (mucopolysaccharidosis IV), Hunter Syndrome (mucopolysaccharidosis II), Farber Disease, Acid Lipase Deficiency, Krabbe Disease, and Sly Syndrome (mucopolysaccharidosis VII). In each of these diseases, lysosomes are unable to degrade a specific compound or group of compounds because the enzyme that catalyzes a specific degradation reaction is missing from the lysosome, is present in low concentrations in the lysosome, or is present at sufficient concentrations in the lysosome but is not functioning properly.
Lysosomal storage diseases have been studied extensively and the enzymes (or lack thereof) responsible for particular diseases have been identified. Most of the diseases are caused by a deficiency of the appropriate enzyme in the lysosome, often due to mutations or deletions in the structural gene for the enzyme. For some lysosomal storage diseases, the enzyme deficiency is caused by the inability of the cell to target and transport the enzymes to the lysosome, e.g., I-cell disease and pseudo-Hurler polydystrophy.
Lysosomal Storage diseases have been studied extensively and the enzymes (or lack thereof) responsible for particular diseases have been identified (Scriver, Beaudet, Sly, and Vale, eds., The Metabolic Basis of Inherited Disease, 6th Edition, 1989, Lysosomal Enzymes, Part 11, Chapters 61-72, pp. 1565-1839). Within each disease, the severity and the age at which the disease presents may be a function of the amount of residual lysosomal enzyme that exists in the patient.
The lysosomal targeting pathways have been studied extensively and the process by which lysosomal enzymes are synthesized and transported to the lysosome has been well described. Komfeld, S. (1986). “Trafficking of lysosomal enzymes in normal and disease states.” Journal of Clinical Investigation 77: 1-6 and Kornfeld, S. (1990). “Lysosomal enzyme targeting.” Biochem. Soc. Trans. 18: 367-374. Generally, lysosomal enzymes are synthesized by membrane-bound polysomes in the rough endoplastic reticulum (“RER”) along with secretory glycoproteins. In the RER, lysosomal enzymes acquire N-linked oligosaccharides by the en-bloc transfer of a preformed oligosaccharide from dolichol phosphate containing 2 N-acetylglucosamine, 9-mannose and 3-glucose. Glycosylated lysosomal enzymes are then transported to the Golgi apparatus along with secretory proteins. In the cis-Golgi or intermediate compartment lysosomal enzymes are specifically and uniquely modified by the transfer of GlcNAc-phosphate to specific mannoses. In a second step, the GlcNAc is removed thereby exposing the mannose 6-phosphate (“M6P”) targeting determinant. The lysosomal enzymes with the exposed M6P binds to M6P receptors in the trans-Golgi and is transported to the endosome and then to the lysosome. In the lysosome, the phosphates are rapidly removed by lysosomal phosphatases and the mannoses are removed by lysosomal mannosidases (Einstein, R. and Gabel, C. A. (1991). “Cell- and ligand-specific deposphorylation of acid hydrolases: evidence that the mannose 6-phosphate is controlled by compartmentalization.” Journal of Cell Biology 112: 81-94).
The synthesis of lysosomal enzymes having exposed M6P is catalyzed by two different enzymes, both of which are essential if the synthesis is to occur. The first enzyme is UDP-N-acetylglucosamine: lysosomal enzyme N-Acetylglucosamine-1-phosphotransferase (“GlcNAc-phosphotransferase”) (E.C. 2.7.8.17). GlcNAc-phosphotransferase catalyzes the transfer of N-acetylglucosamine-1-phosphate from UDP-GlcNAc to the 6 position of 1,2-linked mannoses on the lysosomal enzyme. The recognition and addition of N-acetylgluocosamine-1-phosphate to lysosomal hydrolases by GlcNAc-phosphotransferase is the critical and determining step in lysosomal targeting. The second step is catalyzed by N-acetylglucosamine-1-phosphodiester-N-Acetylglucosaminidase (“phosphodiester α-GlcNAcase”) (E.C. 3.1.4.45). Phosphodiester α-GlcNAcase catalyzes the removal of N-Acetylglucosamine from the GlcNAc-phosphate modified lysosomal enzyme to generate a terminal M6P on the lysosomal enzyme.
GlcNAc-phosphotransferase is an enzyme that contains six subunits; α2β2γ2. The α and β subunits are encoded on a single mRNA and proteolytically cleaved after translation. The γ subunit is encoded on a separate mRNA molecule. Removal of the transmembrane domain from the α/β polyprotein results in a soluble form of the enzyme. This soluble form of GlcNAc-phosphotransferase facilitates a quicker and simpler purification scheme that reduces or eliminates the need for detergents to extract the non-soluble GlcNAc-phosphotransferase from membrane fractions. However, notwithstanding the ease of purification the recombinant soluble GlcNAc-phosphotransferase was not efficiently subject to post-translational proteolytic cleavage when expressed in mammalian cells such as 293T cells and CHO-K1 cells. Uncleaved forms of α/β/γ GlcNAc-phosphotransferase had poor GlcNAc phosphotransferase activity.
To solve this problem, the present inventors have discovered that by interposing a unique proteolytic cleavage site between the α and β subunits in the GlcNAc polyprotein, the polyprotein is cleaved and when expressed with the γ-subunit effectively phosphorylates an enzyme substrate.
In addition, the present inventor has discovered, quite unexpectedly, that the α and β subunits alone is catalytically active. Furthermore, the absence of the γ-subunit results in loss of substrate specificity to only those lysosomal enzymes targeted via the mannose-6-phosphate targeting systems, e.g., acid α-glucosidase, acid β-galactosidase, β-hexaminidase, and others as described herein. This loss of substrate specificity allows the soluble GlcNAc-phosphotransferase containing the α and β tetramer to effectively phosphorylated any glycoprotein having an appropriate acceptor oligosaccharide, for example, mannose-6 through mannose-9 isomers (Baranski et al (1990) Cell 63:281-291).